The centres call people claiming that they are acting for Microsoft and that they have been “alerted” to problems with the user’s computer. Photograph: Alamy

Microsoft has finally ejected an Indian company which was one of its "Gold" partners from the scheme after deciding it was taking part in a "fake virus" telephone support scam.

But Microsoft has been criticised for slow action, after one IT consultant said he had been trying to bring the activities of Comantra, based in Kolkata, to its attention for more than six months, and says that the US software giant has been warned about similar companies' actions for at least 18 months.

The Guardian detailed last year how call centres in India are used to call people in the UK, Australia, New Zealand, US and Canada by people claiming that they are acting for "Microsoft" or "Windows", and that they have been "alerted" to problems with the user's computer. They then direct them to Event Viewer, a console program on the Windows operating system, which shows messages that to the average user look alarming – but are in fact harmless internal messages used by the system.

The callers then use remote login software to gain access to the user's computer, and "fix" the problems; in some cases they also install programs including antivirus and even entire operating system upgrades. They then demand a fee.

The UK police have determined that the actions constitute fraud because there is nothing wrong with the users' machines, and that the money is extracted under false pretences. Scores of people who have contacted the Guardian after being scammed have demanded, and usually received, repayments from banks and credit card providers.

Information passed to the Guardian by sources familiar with the volume of transactions made by the fake support call companies suggest the scam is worth roughly £2m per year. It has grown in size over the past two years, and is believed to be masterminded by one man based in the city of Kota in Rajasthan.

Now Microsoft has thrown Comantra, a website registered to an address in Kolkata, India, out of its "Gold Partner" membership scheme. "Gold Partners" are the top level of Microsoft's "solution partners" and get priority listing in Microsoft's online directory of companies to work with: to achieve the status they have to demonstrate particular levels of technical expertise to Microsoft, plus "the proven ability to deliver solutions featuring Microsoft products", the company says. There is also an annual fee of about £2,900. However, Microsoft does not audit the business practices of companies that gain its accreditation.

Comantra had not replied to a request for a response by the time of publication.

The Guardian has been contacting Microsoft since June 2010 pointing out India-based companies which claim to have Microsoft Partner and Gold Partner status which take part in the "cold call virus scam", and received repeated statements saying that it does not approve of them.

The UK police have in the past taken action to shut down sites used to make people think that the "support calls" come from valid organisations.

But the Guardian's investigations have found that they are built using a template and that as quickly as one is closed, another one or two are opened. Staff are thought to work off scripts from a small number of call centres from which they can claim to be working for a large number of such sites.

A British IT consultant who complained to Microsoft after being contacted repeatedly by Comantra says the US company has been too slow to act.

"I don't think Microsoft has taken this at all serious. It knew about the problem for at least 18 months, through posts on its own internet forums. I reported the issue in March 2011 to their security guru Monika Josi, and I think she has done nothing about it."

He declined to give his name, because, he said: "These scammers don't have any scruples. I don't want to have to worry about whether they would track me down. But they target vulnerable people by calling during the day, when you'll get retired people or carers who won't know what this is about, and who will be taken in if they're told the call is from 'Microsoft' or 'Windows'."

Comantra's site had removed any mention of its former Gold Partner status by Thursday morning. But it was still showing logos for the antivirus services McAfee and Norton.

In a statement, Microsoft said: "We were made aware of a matter involving one of the members of the Microsoft Partner Network acting in a manner that caused us to raise concerns about this member's business practices. Following an investigation, the allegations were confirmed and we took action to terminate our relationship with the partner in question and revoke their Gold status.

"There are no circumstances under which we would ever allow partners or any other organisations to pose as Microsoft. We view matters such as these extremely seriously and take immediate action if such behaviour is brought to our attention and found to be the case. We continue to encourage customers to exercise caution from scams and follow the guidance found at microsoft.com/security."